It's mq4 language program contain certain of trading logic which describe how to decide whether go long or short, including logic to close the order at certain profit or loss automatically.
This program compiled become ex4 file that executable on mt4 trading terminal.
Anybody else?

Think of it as an attachment to your MT4 platform that makes trading decisions for you based on someone else's, or your own, programed trading method. To sum it up in one word, EA's are a gimmick. They will not make you money on their own without constant tweaking of the settings by you in order to accommodate changing market conditions. Just as there is no one manual trading strategy that works best all the time for all market conditions, there's also no one EA that can profitably trade for you at all times in all market conditions. Most of them claim to have a secret weapon, a certain time at which they trade really well, but that's not how the forex market works. The forex market is run by human emotions and not robots. You should only be interested in forex trading for yourself, although I guess if you wanted to make money on selling useless gimmicks to the uneducated masses you could also do that as well.

Notice that EA in your original posting turned blue. That's an auto-link to the FPA's glossary.

Some people do refer to EAs as trading robots. Like any robot, good programming may not always produce the desired results, but will at least keep it from going berserk. Bad programming (or flawed logic in otherwise good programming) can send an EA rampaging through your account balance.

Pretty much, it's just an automated set of trading rules. If certain conditions are met, it opens a trade. When other conditions are met, it closes a trade. It can make (and lose!) money while you sleep.

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